CVE-2026-63940

Guest-to-host memory corruption via zero-length Port I/O

Published Jul 19, 2026 · Updated Jul 19, 2026

Integer underflow in Linux Kernel KVM SEV allows guest OS users to corrupt host memory or panic the host through crafted Port I/O. In kvm_sev_es_string_io(), a zero size or count reaches scratch-area setup, enabling unsigned underflow, zero-size allocation, out-of-bounds access, and division by zero. Execution inside an SEV-ES or SEV-SNP guest is required; a crafted GHCB VMGEXIT can corrupt host memory or panic the hypervisor, interrupting all co-hosted workloads.

CVSS severity9.3
Critical
EPSS probability0.18%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Integer underflow in Linux Kernel KVM SEV allows guest OS users to corrupt host memory or panic the host through crafted Port I/O. In kvm_sev_es_string_io(), a zero size or count reaches scratch-area setup, enabling unsigned underflow, zero-size allocation, out-of-bounds access, and division by zero. Execution inside an SEV-ES or SEV-SNP guest is required; a crafted GHCB VMGEXIT can corrupt host memory or panic the hypervisor, interrupting all co-hosted workloads.

The record

CVE
CVE-2026-63940
Published
Jul 19, 2026
Updated
Jul 19, 2026
Vendor
The Linux Kernel Organization
Product
Linux Kernel
Classifications
CWE-130, T1499.004
Attack vector
local
Privileges
Unavailable

Timeline

How it unfolded

  1. Jul 19, 2026CVE publishedPublication date reported by the CVE source.
  2. Jul 19, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=291bd20d5d88814a73d43b55b9428feab2f28094 <2254972d4d69e279ba4e87bf0968eb08ad0d3c92
  2. Affected versionversion=291bd20d5d88814a73d43b55b9428feab2f28094 <3988bd2723de407ae90fa7a6f6029b4e60238c58
  3. Affected versionversion=291bd20d5d88814a73d43b55b9428feab2f28094 <3b6035bc6bff20e89752ce4358bc4c9a9d5883f2
  4. Affected versionversion=291bd20d5d88814a73d43b55b9428feab2f28094 <c30cde934c7813b4e3069765dac64ce3d31e34f2
  5. Affected versionversion=5.11

What conditions does exploitation require?

Attack vectorlocal
Required privilegesUnavailable in this record.

What is affected?

The Linux Kernel Organization · Linux Kernelversion=291bd20d5d88814a73d43b55b9428feab2f28094 <2254972d4d69e279ba4e87bf0968eb08ad0d3c92; version=291bd20d5d88814a73d43b55b9428feab2f28094 <3988bd2723de407ae90fa7a6f6029b4e60238c58; version=291bd20d5d88814a73d43b55b9428feab2f28094 <3b6035bc6bff20e89752ce4358bc4c9a9d5883f2; version=291bd20d5d88814a73d43b55b9428feab2f28094 <c30cde934c7813b4e3069765dac64ce3d31e34f2; version=5.11

Published CVSS scores

9.3kernel.orgCVSS:3.1/AV:L/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H
7.0Red HatCVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectorlocal
PrivilegesUnavailable
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-130Improper Handling of Length Parameter Inconsistency
T1499.004Application or System Exploitation

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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